I think you are doing a fantastic job with Zero. He's survived this long just because of all you've done for him. You just have a complicated case with him. And it can be stressful to deal with at times. I was so stressed with my Zeus and his stasis. I had to remind myself that I was doing my best with him, and tried to just focus on what I needed to do each day, with the meds and syringe feeding. I would think we had it kicked, then he would get it again, and again. It wasn't until I realized that it was the pellets causing the stasis to keep reoccurring and thus stopping feeding them, that he finally got better and hasn't had problems with stasis since. But it's a learning curve with rabbits, and it takes trial and error sometimes.
I came across this on a UK rabbit forum.
"One of Barney's issues is gas and the vet said his gut stimulant (metaclop) should help the gut push it through. One of the reasons it collects is because of his slow gut - food just sits there. Perhaps a gut stimulant could help your bun, if gas is an issue? Perhaps worth discussing with the vet after the outcome of the tests."
Maybe keeping Zero on a gut motility med, will help control the gas and bloating problems. Maybe he also needs a hay that has a lot more fiber, to keep his gut moving more quickly. I think the hays with the highest fiber would be straw, like oat, wheat, and barley straw(hopefully with all the grain sifted out and only stalks left). There would be no nutritional value, but he would be getting a ton of fiber. The other option would be to get a very mature grass hay that is mostly thick stems. You would need to make sure there weren't any seed heads though.
They really seem to have a better handle on rabbit health in the UK. I think because rabbits are a much more common pet over there, so there are more vets that specialize in rabbits. Maybe your vet could consult with a rabbit specialist in the UK. I know that Francis Harcourt Brown is considered, probably, the number one specialist with rabbits.